1. Field of the Invention
The present invention relates to a chromatic dispersion compensating apparatus for further increasing the capacity, the speed, and the distance of an optical communications system hereafter.
2. Description of the Related Art
With the sharply growing network use in recent years, the demand for further increasing the capacity of a network has been rising. Currently, a wavelength multiplexing (WDM) optical transmission system on a basis of a transmission rate of 10 Gb/s per channel has been put into practical use. Hereafter, a further increase in the capacity is expected to be required, and an ultrahigh-speed transmission system of 40 Gb/s or faster per channel is demanded from the viewpoints of frequency use efficiency and cost. In an ultrahigh-speed transmission system, wavelength degradation caused by dispersion of a transmission line must be compensated with high accuracy.
In an optical transmission system having a transmission rate of 10 Gb/s or faster, a chromatic dispersion tolerance is very small. For example, the chromatic dispersion tolerance of a 40-Gb/s NRZ system is equal or smaller than 100 ps/nm. In the meantime, for a terrestrical optical transmission system, span length are not always uniform. In the case of a system using a 1.3-μm zero dispersion single mode fiber (SMF) of approximately 17 ps/nm/km, a chromatic dispersion tolerance is exceeded if the length differs only several kms. However, in an optical fiber network possessed by a communications carrier, most span length and chromatic dispersion values are not accurately grasped at present. Additionally, since a chromatic dispersion value changes with time depending on a fiber temperature, stress, etc., a dispersion compensation amount for each span must be adjusted not only at the start of system operations but also in system use while strictly monitoring a chromatic dispersion amount. For example, a temperature change of 100° C. occurs on a 500-km DSF (Dispersion Shifted Fiber) transmission line, its chromatic dispersion change amount becomes approximately 105 ps/nm that is almost equal to a chromatic dispersion tolerance of a 40-Gb/s NRZ signal.(chromatic dispersion change amount)=(temperature dependency of a zero dispersion wavelength)×(temperature change amount of a transmission line)×(dispersion slope of the transmission line)×(transmission distance)=0.03 (nm/° C.)×100 (° C.)×0.07 (ps/nm2/km)×500 (km)=105 ps/nm
Therefore automatic dispersion compensation is essential for a system using not only an SMF transmission line, but also a 1.55-μm zero dispersion shifted fiber (DSF) or an NZ-DSF transmission line.
Furthermore, when a wavelength-division multiplexed(WDM) signal is transmitted, a dispersion slope as well as chromatic dispersion must be considered.
FIG. 1 exemplifies the configuration of a WDM transmission system. FIG. 2 shows a change in a chromatic dispersion amount of a transmission line due to various change factors.
In the configuration shown in FIG. 1, optical signals of respective wavelengths are transmitted from optical transmitters #1 to #n of a transmitting end station device, and coupled by an optical multiplexer. The multiplexed optical signal is amplified and output by an optical post-amplifier. When the process for amplifying the optical signal is performed by the optical post-amplifier, dispersion compensation is made for the optical signal by a transmission dispersion compensator in the transmitter, whose dispersion compensation amount is fixed or variable. The optical signal which propagates over a fiber transmission line is amplified so that a transmission line loss is compensated by an optical inline amplifier, which exists partway of the fiber transmission line. Additionally, chromatic dispersion that the optical signal undergoes as a result of propagating over the transmission line is compensated by an inline dispersion compensator, when the amplification is made by the optical inline amplifier. The dispersion compensation amount of the inline dispersion compensator may be fixed or variable. Furthermore, the optical signal is propagated over the fiber transmission line via an inline amplifier, and input to an optical receiver.
In the optical receiver, the propagated optical signal is amplified so that its attenuation is compensated. At this time, dispersion compensation also at the receiver side is made by a reception dispersion compensator in the receiver. Then, the propagated optical signal is split into respective wavelengths by an optical demultiplexer. For example, variable dispersion compensators remove residual dispersion from the optical signals of the demultiplexed wavelengths, and the signals are received by optical receivers #1 to #n. Here, the reason why the variable dispersion compensators are enclosed by brackets is that they are not always necessary. Whether or not the variable dispersion compensator are included can be determined by a designer depending on the details of a design. If a constituent element is enclosed by brackets also in the subsequent configuration drawings, it means that the constituent element is not always required to be included at the discretion of a designer.
For a temperature change in the chromatic dispersion of an optical signal, a chromatic dispersion characteristic (a) shifts to (c) according to a temperature change (approximately 0.03 nm/° C.) in a zero dispersion wavelength as shown in FIG. 2. In this case, a dispersion slope does not change. Additionally, if a transmission distance is different, the chromatic dispersion characteristic (a) changes to (b). In this case, also the dispersion slope changes with the dispersion amount. For an actual transmission line fiber (and a dispersion compensation fiber (DCF)), the chromatic dispersion value ((a)→(c)), and the dispersion slope ((a)→(d)) have variations due to a problem of a fiber manufacturing ability, even if the length of a transmission line is the same.
As a means for compensating for chromatic dispersion and a dispersion slope, the following methods are considered.    (a) Implementing a broadband variable dispersion compensator that can independently vary a chromatic dispersion amount and a dispersion slope amount, and making dispersion compensation simultaneously for signals of all of wavelengths.    (b) Independently arranging a broadband variable dispersion compensator that can vary a chromatic dispersion amount, and a broadband variable dispersion slope compensator that can vary a dispersion slope amount, and making dispersion compensation collectively for signals of all of wavelengths.    (c) Independently arranging a broadband variable dispersion compensator that can vary a chromatic dispersion amount, and a fixed dispersion slope compensator whose dispersion slope amount compensates a slope amount of a transmission line, and making dispersion compensation simultaneously for signals of all of wavelengths.    (d) Individually arranging for each channel a variable dispersion compensator that can vary a chromatic dispersion amount, and making dispersion compensation.
The most important point in the methods (a) to (d) is the practicability of a variable dispersion compensator.
FIG. 3 shows a VIPA (Virtually Imaged Phased Array) as an example of a variable dispersion compensator. As documents about a VIPA, please see M. Sirasaki et al., “Variable Dispersion Compensator Using the Virtually Imaged Phased Array (VIPA) for 40-Gbit/s WDM Transmission System”, ECOC 2000, Post-deadline paper 2.3., etc.
In a dispersion compensator using a VIPA, a dispersion compensation amount can be successively changed in a range from −800 ps/nm to +800 ps/nm by moving a three-dimensional mirror in the direction of an x axis.
FIG. 4 shows a transmittance characteristic and a group delay characteristic of a VIPA variable dispersion compensator.
The transmittance characteristic shown in an upper portion of this figure exhibits a periodical wavelength dependence of transmittance in a VIPA. Accordingly, a design must be made so that optical signals of respective wavelengths of wavelength multiplexed light (WDM light) pass through a high portion of the transmittance, namely, a transmittance window. Additionally, the figure of the group delay represents that the group delay is periodically given to an optical signal. It is proved from this figure that the slope of the group delay in a portion where the transmittance window is opened is decreasing on the right, and negative dispersion is given to an optical signal that passes through the window.
For example, a VIPA is designed to have a cyclic structure where a transmission characteristic has a frequency interval (free spectral range: FSR) of 200 GHz (wavelength interval is 1.6 nm), and advantageous to simultaneously compensate for a WDM signal. However, the VIPA cannot compensate for a dispersion slope. A system implemented by combining a VIPA dispersion compensator and a dispersion compensation fiber in order to collectively compensate for the dispersion compensation and the dispersion slope is proposed by Japanese Patent Application No. 2000-238349.
FIG. 5 shows the group delay characteristic of a VIPA variable dispersion compensator in a channel passband.
In a variable dispersion compensator using a VIPA shown in an upper portion of FIG. 5, changes in the slope of a group delay shown in a lower portion of FIG. 5 are obtained by moving a three-dimensional mirror in the direction of an x axis. Dispersion is obtained by differentiation of wavelengths of a group delay. Therefore, simultaneous dispersion compensation can be varied and made depending on need for all of channel bands by moving the three-dimensional mirror.
FIG. 6 exemplifies the configuration of an optical receiver according to a conventional technique.
In the configuration example shown in this figure, a DCF whose dispersion slope amount (a dispersion slope of a transmission line) arranged to compensate for a dispersion slope of the transmission line. Furthermore, chromatic dispersion caused by the transmission line and the DCF is collectively compensated by using a VIPA variable dispersion compensator. As shown in FIG. 4, the VIPA has the periodical structure of 200 GHz intervals in order to secure a transmission band. In a current dense WDM transmission system, 100 GHz channel spacing(wavelength interval of 0.8 nm) is demand. Accordingly, in FIG. 6, a received signal of 100-GHz spacings is separated into even- and odd-numbered channels of 200-GHz intervals by using an interleaver, and dispersion compensation is simultaneously made by arranging VIPA dispersion compensators respectively for the even- and odd-numbered channels. As shown in FIG. 7, a transmittance window of the interlever is opened in predetermined cycles (200 GHz in this case) A solid line shown in this figure is a window for extracting odd-numbered channels, whereas a dotted line shown in this figure is a window for extracting even-numbered channels. As described above, the interleaver alternately samples a wavelength multiplexed optical signal, and separates the optical signal into even- and odd-numbered channels, so that the channel intervals of the optical signal after being separated are widened.
However, this configuration has a problem stemming from the wavelength dependency of a dispersion slope of a transmission line and a DCF, leading to a difficulty in simultaneous dispersion compensation.
FIG. 8 shows a typical example of a dispersion characteristic on a fiber transmission line.
Mainly on a DCF, a dispersion curve derived from the wavelength dependency of a dispersion slope occurs due to a manufacturing problem (however, an almost linear dispersion characteristic is possessed in a transmission fiber). Accordingly, residual dispersion derived from the wavelength dependency of a dispersion slope occurs on a transmission line and a DCF. In a long-haul transmission, this residual dispersion becomes a value that exceeds the dispersion tolerance of a 40-Gb/s signal. Therefore, simultaneous compensation is difficult with the configuration of FIG. 6 itself.
Also an implementation of a dispersion monitor for detecting a chromatic dispersion amount (and a slope amount), which a transmission line undergoes, is important to realize an automatic dispersion compensating system.
As an example of a dispersion monitor method, there is a method using the intensity of a particular frequency component within a received baseband signal.
FIG. 9 shows a result of detecting the intensity of a 40-GHz component within a received baseband signal of a 40-Gb/s NRZ signal.
Source: Y. Akiyama et al., “Automatic Dispersion Equalization in 40 Gbit/s Transmission by Seamless-switching between Multiple Signal Wavelengths”, ECOC '99, pp. 1-150-151
As is known from a calculation result shown on the left side, the intensity of a 40-GHz component varies with a chromatic dispersion amount, and becomes zero when the dispersion amount is zero. In an experimental result of a 100-km DSF transmission on the right side, the dispersion amount of a transmission line varies with a wavelength. Therefore, the intensity characteristic of a 40-GHz component is obtained in a similar manner as in the calculation result. A zero dispersion wavelength of the transmission line varies with a change in the temperature of the transmission line by approximately 0.03 nm/° C. However, it can be verified that also the minimum point of an intensity monitor of the 40-GHz component varies with that change. It is known that the intensity of a B Hz component is available as a chromatic dispersion monitor for a B b/s modulation signal also with other modulation methods. It is known, for example, when chromatic dispersion is zero, the intensity of a B Hz component becomes a maximum for an RZ signal, and becomes a minimum for an OTDM signal (Japanese Patent Application No. Hei 9-224056).
As another means, a method monitoring a bit error rate characteristic or a Q value, which is detected by each optical receiver, is considered.
To implement a low-cost dispersion monitor in a wavelength multiplexing system, a method arranging a dispersion monitor is important. For example, in the case of (a) or (b) shown in FIG. 2, if chromatic dispersion amounts of at least two signals such as signals of wavelengths at both ends of a signal wavelength band can be detected, a dispersion slope can be learned by extrapolation, and a chromatic dispersion amount of a different signal wavelength can be detected.
Additionally, in the case of (c), the dispersion slope amount of the transmission line does not vary with a temperature change. Therefore, if a chromatic dispersion amount of at least one signal such as a central wavelength signal, etc. of a signal wavelength band can be detected, a chromatic dispersion amount of a different signal wavelength can be detected from the chromatic dispersion amount and the known dispersion slope amount.
Also in the case of (d), a chromatic dispersion amount of a different signal wavelength can be detected by extrapolation, if a chromatic dispersion value of at least one wavelength signal can be detected when a dispersion slope amount (or the length of a transmission line) is known, or if chromatic dispersion values of at least two wavelength signals can be detected when the dispersion slope amount is unknown.
The above described problems of conventional techniques are summarized below.
In an optical transmission system having a transmission rate of 10 Gb/s or faster, a chromatic dispersion tolerance is very small. For example, the chromatic dispersion tolerance of a 40-Gb/s NRZ system is equal to approximately 100 ps/nm or smaller. In the meantime, for the chromatic dispersion of a transmission line, the following change factors exist. If a wavelength-division multiplexed(WDM) signal is transmitted, not only chromatic dispersion but also a dispersion slope must be considered.
(1) Difference in the Length of a Transmission Line
For a terrestrical optical transmission system, lengths of its span length are not always uniform. In the case of a system using a 1.3-μm zero dispersion single-mode fiber (SMF) of approximately 17 ps/nm/km, a chromatic dispersion tolerance is exceeded if the length is different by only several kilometers. However, in an optical fiber network possessed by a communications carrier, most span length and chromatic dispersion values are not accurately grasped at present. As shown in FIG. 2, the chromatic dispersion characteristic (a) changes to (b) if a transmission distance is different. In this case, also the dispersion slope as well as the dispersion amount changes.
(2) Incompleteness of a Slope Compensation Ratio of a Dispersion Compensation Fiber (DCF)
To make dispersion compensation and dispersion slope compensation simultaneously for a wavelength multiplexed signal, a dispersion compensating fiber (DCF) having a dispersion slope rate (dispersion slope coefficient/chromatic dispersion coefficient) that matches a transmission line must be used. However, especially for an NZDSF fiber (such as Enhanced LEAF, TrueWave Plus, TrueWave Classic, etc.) having a small chromatic dispersion coefficient, a DCF that can be manufactured is only a DCF whose slope compensation ratio is as low as 50 to 60 percent.
FIG. 10 shows fluctuations of chromatic dispersion on a transmission line due to dispersion slope variations.
In this figure, to compensate for a dispersion slope characteristic (a) of the transmission line by 100 percent, it is ideal that a DCF matches a characteristic (a)′ of its reverse sign. Actually, however, a slope compensation ratio as high as (a)′ cannot be obtained, and the ratio becomes like (b). As a result, residual dispersion indicated by (c) occurs on the transmission line and the DCF.
(3) Manufacturing Variations of a Chromatic Dispersion Coefficient and a Dispersion Slope Coefficient of a Transmission Line Fiber and a Dispersion Compensating Fiber (DCF)
Since a chromatic dispersion coefficient (chromatic dispersion amount per unit length. The unit is ps/nm/km), and a dispersion slope coefficient (chromatic dispersion slope per unit length. The unit is ps/nm2/km) of a transmission line and a dispersion compensating fiber (DCF) reach the limits of manufacturing accuracy, they have relatively large variations. Therefore, as shown in FIG. 2, the chromatic dispersion amount (the unit is ps/nm. (a)→(c), −(a)→(b)′), and the dispersion slope amount (the unit is ps/nm2, ((a)→(d), −(a)→(d)′)) vary, even if the lengths of the transmission line and the DCF are the same.
(4) Temperature Change in a Zero Dispersion Wavelength of a Fiber
Since the zero dispersion wavelength of a transmission line fiber changes with time depending on a temperature, a dispersion compensation amount for each span must be suitably set while strictly monitoring a chromatic dispersion amount not only at the start of system operations, but also in system use.
For example, if a temperature change of 100° C. occurs on a 600-km transmission line, a chromatic dispersion change amount becomes approximately 108 ps/nm, which is almost equal to the chromatic dispersion tolerance of a 40-Gb/s NRZ signal.(chromatic dispersion change amount)=(temperature dependency of zero dispersion wavelength)×(temperature change amount of transmission line)×(dispersion slope of transmission line)×(transmission distance)=0.03 (nm/° C.)×100° C.×0.06 (ps/nm2 /km)×600 (km)=108 ps/nm
In FIG. 2, the chromatic dispersion characteristic (a) changes to (c) due to a temperature change (approximately 0.03 nm/° C.) of the zero dispersion wavelength. In this case, the dispersion slope does not vary.
(5) Influence of the Wavelength Dependency of a Transmission Line Fiber and a DCF
As shown in FIG. 8, a dispersion curve derived from the wavelength dependency of a dispersion slope occurs due to a problem of design principle also on a transmission line, mainly on a DCF (the transmission fiber has an almost linear dispersion characteristic). Accordingly, residual dispersion derived from the wavelength dependency of the dispersion slope occurs on the transmission line and the DCF. In a long-haul transmission, this residual dispersion becomes a large value that exceeds the dispersion tolerance of a 40-Gb/s signal. This becomes a serious problem when dispersion compensation is made collectively for all of channels.
Measures according to a known technique is as follows. A variable dispersion compensator must be applied to cope with time-varying chromatic dispersion fluctuations in (4). As an example of the variable dispersion compensator, the VIPA shown in FIG. 3 exists. As a method arranging a variable dispersion compensator, there are a method making compensation simultaneously for all of channels by also comprising a slope compensation function, a method making compensation simultaneously for all of channels by combining with a variable or fixed dispersion slope compensator, or a method arranging a variable dispersion compensator for each channel (see Japanese Patent Application No. 2000-238349) is considered.